Amplifier circuit



May 1, 19278. 1,668,151

L. M. HULL AMPLIFIER CIRCUIT Filed Nov. 17. 1926 Patented May 1, 1928.

. UNITED STATES PATENT OFFICE.

.LEWIS M. HULL, OF BOONTON, NEW JERSEY, ASSIGNOR T0 RADIO FREQUENCY LABO- RATORIES INCORPORATED, 0F BOONTON, NEW JERSEY, A CORPORATION OF NEW JERSEY.

. AMPLIFIER CIRCUIT.

Application filed November 17, 1926 Serial 'No. 148,975.

This invention relates to an electrical amplifier circuit for use in association with a four-electric vacuum tube of the type having an emittingcathode, an anode, and two grid electrodes. For manypurposes it is desirable to operate such a vacuum tube according to a method, known in this art, which includes employing the grid G which is next the cathode, or which is the inner grid in a concentric arrangement, as the control element, and giving to the grid G which is next the anode, a positive bias. This method of operating a four-electrode tube makes use of what may, for convenience, be termed the emission-shield effect of the grid G The result is an increase in the effec tive alternating current plate impedance of the tube, due to the fact that the effect of the plate voltage upon the total electric field attracting the space current to the plate is greatly reduced by the-shielding action of the grid G Another effect is the reduction of the effective interelectrode tube capacity between the control grid G, and the plate, and a consequent reduction of undesirable retroactive currents which are fed back from the output circuit to the input circuit, through these tube capacities.

This method of operating a four-electrode vacuum tube as an electrical amplifier does not, in many instances, accomplish of itself a sufficient suppression of these undesirable currents. This invention relates to means for reducing or suppressing these undesirable feed-back currents in a four-electrodc vacuum tube when operated as an electrical amplifier in the manner described.

I have found that by impressing upon the grid, G a voltage having the proper amplitude and phase relation, as will be later explained, to the voltage of the control grid L. I can reduce to any desired extent, or suppress, the undesirable feed-back currents which otherwise flow through the tube capacities.

This may be accomplished in various ways, certain typical ones of which I will now describe in connection with the accompanying drawings.

Figures 1, 2, 3 and 4: are diagrams illustrating certain specific embodiments of my invention.

Fig. 1 shows a four-electrode vacuum tube having emitting-cathode F, anode P, (control) grid G .a second (emission-shield) grid G. and input and output circuits C vL and C,L,, respectively. The input and output circuits may be tuned or not as desired. They are hereshown as being tuned, but in the case of a cascaded amplifier having two or more stages I may use, for example, transformer coupling between the stages, and may tune only the transformer secondaries. This is shown in Fig. 4. In a cascade arrangement, direct or other coupling may equally be used. The input or output circuits may be untuned, whether the audions are cascaded or not. The A. B, and C batteries are arranged as usual. The grid, G is connected to. the filament through coil L,,, which is coupled to coil L in the input circuit. The proper positive bias is imparted to the grid G,, by including more or less of the B battery in this lead, or otherwise.

In this case coil L should be so wound with respect to L,, that the compensating voltage impressed upon Gr, is opposite in phase to the voltage on the grid G The amplitude of the compensating voltage may be adjusted to give the desired suppression of the feed-back currents. This may be done by giving L and L the proper coup ling or 1n other ways. coupling coefficient between these coils are suitable; I have used a coefficient of coupling equal to .5 with satisfactory results. The operation of my circuit is not dependent upon the use of substantially unity coupling between these coils, as is the case with certain other circuits, for example that known as the neutrodyne. In some instances I find it advantageous to employ capacity Cg, connected between the grid G and the plate, as in Figures 1, 3 and 4, or between the grids G, and G,, as in Fig. 2.

An arrangement like that shown in Fig. 1 produces other beneficial results in addition to the suppression of feed-back currents. An important advantage arises from the secondary control effect exerted by the grid G in virtue of the coupling between coils L and L This control effect is of such a nature that more constant amplification is ob tained over a range of Wavelengths.

In Fig. 2 the compensating voltage is applied to the grid G, by means of coil L coupled to coil L of the output circuit. The coil L should be so wound with respect to L that the voltage upon Gr is in phase with the Various values of the voltage upon G, at resonance; that is, the voltage upon G is opposite in phase to the voltage upon the anode, P. The remainder of the circuit may appropriately be arranged as before.

In this arrangement, in addition to the suppression of feed-back currents, the secondary control effect of grid Gr tends to increase the amplification of the vacuum tube.

Fig. 3 shows an arrangement for applying the compensating voltage to the grid Gr by means of coil L in series with coil L ot the input circuit, but not necessarily coupled thereto. A by-pass condenser C is then employed, and the tuning condenser C, may be connected across both coils L and L, as shown.

Fig. i has already been referred to in describing Fig. 1. It shows one suitable method of cascading a plurality of vacuum tube amplifiers arranged according to this invention. The circuit of Fig. 1 is here illustrated in cascade by way of example only. The circuits as shown in the other figures may also be cascaded.

The circuits herein shown are of course merely typical of the means which i have found by experiment to be appropriate tor carrying out my invention, by impressing the proper compensating voltage upon the grid G Equivalent circuits for accomplishing this result, or variations of the circuits shown, fall within my invention.

1 claim:

1. An electrical amplifier comprising a vacuum tube having an emitting cathode, an anode, a control grid, and a second grid, an input circuit connected between said cathode and said control grid, an output circuit connected between said cathode and said anode, and means associated with one of said circuits for impressing upon said second grid a compensating voltage of the proper phase and amplitude to oppose "feedback currents flowing through the capacities of said vacuum tube between said anode and said control grid.

2. An electric amplifier comprising a vacuum tube having an emitting cathode, an anode, a control grid, and a second grid, an input circuit connected between said cathode and said control grid, an output circuit connected between said cathode and said anode,

and a coil connected between said cathode and said second grid, and coupled to a coil in one of said circuits in such away as to impress upon said second grid a compensating voltage of the proper phase and amplitude to oppose feedback currents flowing through the capacities of said vacuum tube between said anode and said control grid.

3. An electric amplifier comprising a vacuum tube having an emitting cathode, an anode, a control grid, and a second grid, an input circuit connected between said cathode and said control grid, an output circuit connected between said cathode and said anode, and a coil connected between said cathode and said second grid and coupled to a coil in said input circuit in such a way as to impress upon said second grid a compensating voltage of the proper phase and amplitude to oppose teed-back currents flowing through the capacities of said vacuum tube between said anode and said control grid.

at. An electric amplifier comprising a vacuum tube having an emitting cathode, an anode, a control grid, and a second grid, an input circuit connected between said cathode and said control grid, an. output circuit connected between said cathode and said anode, and a coil connected between said cathode and said second grid and so cou pled to a coil in said input circuit as to impress a compensating voltage upon said second grid to oppose teed-back currents flowing through the capacities of said vacuum tube between said anode and said control grid.

5. An electric amplifier comprising a vacuum tube having an emitting cathode, an anode, a control grid, and a second grid, an input circuit connected between said cathode and said control grid, an output circuit connected between said cathode and said anode, a coil connected between said cathode and said second grid. and so coupled to a coil in said input circuit as to impress upon said second grid a compensating voltage to oppose feed-baclr currents flowing through the capacities of said vacuum tube, and a capacity connected between said second grid and said anode.

lln testimony whereof, I affix my signature.

LEWIS M. HULL. 

